1. Field of the Invention
The present invention relates to an X-ray generating apparatus that generates X-rays, and a method of driving an X-ray tube that the X-ray generating apparatus includes.
2. Description of the Related Art
Conventionally, X-ray tubes that generate X-rays are configured to use a reflection type target or a transmission type target. Both of these configurations cause electrons emitted from an electron source to collide with a target to generate X-rays from the target by means of the collision energy. The generated X-rays pass through a transmission window provided in the X-ray tube and are emitted to outside.
Generally, an X-ray tube includes an electron source that emits electrons, a lens electrode that focuses electrons emitted from the electron source, and a target that generates X-rays when electrons collide therewith. An electron source includes a grid electrode that draws out electrons from an electron emission source such as a filament (in the case of a directly heated electron source) or a cathode electrode (in the case of an indirectly heated electron source). A required voltage is supplied to the filament, the cathode electrode, the grid electrode, the lens electrode, the target or the like, respectively, from an external drive circuit.
Further, for example, in an X-ray tube that uses a transmission type target as a target, when electrons collide with the transmission type target, X-rays are radiated in all directions from the collision region. Therefore, a rearward shielding member is provided on the lens electrode side of the transmission type target and a forward shielding member is provided on the transmission window side of the transmission type target. By providing the rearward shielding member and the forward shielding member, radiation of X-rays in unnecessary directions is suppressed.
Normally, in an X-ray tube, it is necessary to control the order of applying the respective voltages to the electron emission source, the grid electrode, the lens electrode and the target when generating X-rays. Furthermore, when stopping the generation of X-rays, it is necessary to control the order of stopping application of each voltage to the electron emission source, the grid electrode, the lens electrode and the target.
For example, when emitting X-rays, a filament or a heater is preheated by applying a predetermined voltage thereto, and a voltage (first voltage) for not allowing thermal electrons emitted from the electron emission source to reach the target is applied to the grid electrode and lens electrode. Subsequently, at a time of actually generating X-rays, first, a predetermined high voltage is applied to the target. Next, a voltage (second voltage) for focusing electrons emitted from the electron emission source is applied to the lens electrode, and finally a voltage (second voltage) for drawing out electrons from the electron emission source is applied to the grid electrode. In this connection, preheating of the filament or the heater is necessary in order to stably emit X-rays simultaneously with the input of an instruction to irradiate X-rays (application of a second voltage to the grid electrode).
In contrast, when stopping the emission of X-rays, the applied voltage is switched from the second voltage to the first voltage in the order of the grid electrode and the lens electrode, thereafter the application of a high voltage to the target is stopped, and finally application of a voltage to the filament or heater is stopped.
In this case, for example, if the second voltage is applied to the grid electrode in a state in which the high voltage is not being applied to the target, electrons that are drawn out from the electron emission source collide with a member other than the target (such as the lens electrode or the rearward shielding member). There is thus a risk that unwanted X-rays will be unintentionally generated. At that time, if the second voltage is being applied to the lens electrode, since the electrons that are drawn out from the electron emission source are focused, collision of the electrons with the rearward shielding member can be suppressed. However, in that case, most of the electrons drawn out from the electron emission source flow into the lens electrode, and hence an overcurrent flows through the lens electrode or a drive circuit thereof. If a state in which an overcurrent flows through the lens electrode or a drive circuit continues for a long time, there is a risk that the lens electrode or the drive circuit will be damaged.
Therefore, according to the conventional X-ray generating apparatus, to enable control of the order of applying and the order of stopping application of voltages to a filament (or heater), a grid electrode, a lens electrode and a target of an X-ray tube, each voltage is generated by an independent drive circuit, respectively.
Japanese Patent Application Laid-Open No. 2002-299098 discloses an X-ray generating apparatus in which a filament or a heater that heats a cathode electrode that is used as an electron emission source is preheated, and thereafter a voltage is applied to a grid electrode.
As described above, according to the conventional X-ray generating apparatus, the respective voltages applied to the filament (or heater), the grid electrode, the lens electrode and the target of an X-ray tube, are generated by independent drive circuits, respectively. Therefore, the scale of the drive circuits of an X-ray tube is large, and this has been a factor that has inhibited a reduction in the size of X-ray generating apparatuses.
An object of the present invention is to provide an X-ray generating apparatus that can be reduced in size compared to conventional X-ray generating apparatuses, and a method of driving an X-ray tube.